Power supply filters, and most particularly anti-interference filters, are lowpass filters used to protect equipment inside or outside Faraday cages from radio frequency disturbances. They are installed, in particular, where power supply cables coming from the mains (public utility or otherwise) penetrate into the cage, e.g. for the purpose of powering equipment to be found inside the cage (with such power mains being referred to herein as "power networks"). Such feedthroughs can facilitate electromagnetic leakage to the outside. Such leakage can have two major consequences, amongst others:
damaging equipment outside the cage; and
giving rise to leakage of secret information in the form of radio signals that can be picked up by external antennas (this consequence is damaging particularly in military applications): to avoid such leakage, anti-interference electrical filters are used, and may be referred to as "anti-eavesdropping filters".
Anti-interference filters are thus essential to enable a Faraday cage to be effective, and for that purpose they must attenuate signals at frequencies that lie outside their passbands. In the same way as it is necessary to verify the screening performance of the cage, it is also necessary to verify the performance of an anti-interference filter. This is to verify whether an operating filter is performing its function properly, i.e. if an outlet voltage from the filter is sufficiently attenuated relative to an inlet voltage outside its passband to prevent any type of leakage.
Such performance evaluation is particularly important since filter components (inductors, capacitors, . . . ) age poorly, degrading over time and thus running the risk of reducing the protection provided by the filter and thus by the cage itself.
The performance of such filters is thus evaluated in conventional manner during maintenance periods that take place once every one or two years, or whenever a defect is detected. For evaluation purposes, the filter must be disconnected from the power network, its inlet connected to a generator and its outlet to a receiver. External transmission and reception means must thus be brought on site. The test is then performed by reconstituting the response of the filter over the entire frequency band, i.e. by reconstituting its transfer function
Such conventional procedures are inconvenient and lengthy. Throughout the duration of the test, the user of the equipment situated inside the cage is no longer supplied with electricity, so the equipment must be turned off. In addition, the test equipment is bulky and inconvenient to install.
Furthermore, to reconstitute the frequency response of a filter, it is necessary to perform tests at different frequencies, which is fiddly, particularly since the test must be repeated for each of the filters present on the cage.
Finally, if the result of the test is to be displayed inside the cage to inform a user, then it is necessary to transmit information via an electrical cable entering the cage. Unfortunately, that cable itself constitutes an additional source of leakage so it too requires a filter. In practice, the result of the test is therefore never sent to a user inside the cage.
The object of the present invention is thus to provide a system for evaluating the performance of a filter that enables the transfer function of the filter to be verified simply and quickly on user command or on a permanent basis and without disconnecting the power network.